The present invention relates to a process and system for treating wastewater in a constructed wetland facility.
The use of constructed wetlands for the treatment of wastewater is becoming an increasingly popular option especially in rural or lightly developed areas. Constructed wetlands use wetland plants and bacteria to remove pollutants from the wastewater.
Existing constructed wetlands, including surface water and subsurface flow systems, use wetland plants and atmospheric diffusion to transfer oxygen into the water. At the inlet to the constructed wetlands, the high concentration of organic matter in the wastewater provides a rich energy source for biological activity. This activity is so vigorous that any oxygen transferred into the water is immediately consumed resulting in an essentially oxygen-free (anaerobic) zone. The organic material is consumed by the biological activity so that, as the wastewater flows through the wetland, the concentration of organic matter is rapidly reduced. The reduction in the energy source represented by the organic matter reduces the rate of microbiological activities such that an anoxic regime is formed wherein the oxygen transferred into the water is not necessarily immediately consumed but the water is still oxygen starved. The plants and water surface can provide natural aerobic zones within the constructed wetland, especially in the anoxic zone. These aerobic zones provide for efficient conversion of ammonia or ammonium ions into nitrates. The nitrates are bacteriologically reduced to nitrogen gas but such microbiological activity is inhibited by the presence of free oxygen in the aerobic zones and the anoxic zones.
A number of processes and systems have been developed to increase the aerobic activity in a constructed wetland to improve the conversion of ammonia/ammonium to nitrates. For example, U.S. Pat. No. 5,893,975 discloses a subsurface flow constructed wetland including a wastewater treatment system having a flow intake, a pretreatment nutrient addition chamber and a wastewater flow divider. The flow divider further has a compressed air aerator in the bottom thereof. An object of the invention of U.S. Pat. No. 5,893,975 is to install a supplemental air diffuser system along the bottom of the influent distribution chamber therein to facilitate initial precipitation of sludge and to prevent potential septic and/or anaerobic conditions at the initial point of treatment to significantly enhance system performance.
U.S. Pat. No. 6,126,827 discloses a wastewater treatment system wherein the liquid component is subjected to a two-part treatment wherein the first part of the treatment is carried out on a sequencing batch reactor and the second part is carried out in at least one reed bed. The wastewater entering the reed bed is comparable in quality to secondary treated wastewater except that a substantial amount of total nitrogen has also been removed.
U.S. Pat. No. 6,159,371 discloses constructed wetlands, utilizing a plurality of cells, in which nitrification and denitrification occurs simultaneously at low flow rates and lower temperatures. The constructed wetland provides improved remediation in a shorter period of time.
U.S. Pat. No. 6,200,469 B1 discloses a system for removing pollution from water, utilizing a subsurface constructed wetland system using forced bed aeration and variable water levels, to promote greater root depth and better root structure of the wetland vegetation, and to establish staged anaerobic and aerobic zones within the system.
A system combining a wetlands and a recirculating sand filter in series has been developed to enhance nitrogen removal. However, this system requires the addition of a second nutrient feed stream after the sand filter.
None of the prior art systems provide for denitrification of the wastewater in a highly anaerobic, nutrient rich zone near the inlet of the constructed wetlands. It is a hallmark of the current invention to provide for denitrification of wastewater in the anaerobic nutrient rich zone at the inlet to the constructed wetland.